稀土元素Gd对Al-Si-Mg铸造合金微观组织和力学性能的影响

为了系统地研究稀土Gd对铸造Al-Si-Mg(A357)合金组织和性能的影响,采用OM,SEM,EPMA,XRD,DSC,TEM及拉伸实验等方法对不同Gd含量A357合金进行研究。结果表明:Gd的添加可以细化A357合金的晶粒并减小二次枝晶间距。此外,Gd可以有效地细化合金中的共晶硅,但是对片状共晶硅的形貌影响不大。晶粒和共晶硅的细化及二次枝晶间距的减小使添加Gd后的A357合金的力学性能有了显著的提高。其中,A357-0.5Gd(质量分数/%)合金热处理态抗拉强度为355MPa,相对于未添加Gd元素的A357合金提高了37MPa。当Gd质量分数为1.0%时,尽管组织得到进一步细化,但是大量粗大Al 2Si 2Gd第二相的形成导致了合金力学性能的下降。同时对Gd的细化机制进行探究,结合TEM分析结果可以推断,Gd变质处理后共晶硅上的孪晶密度并不足以引起共晶硅形貌的转变,使得Gd变质效果较弱。而Gd对共晶硅的细化作用可能与Gd增加成分过冷以及形成纳米相阻碍共晶硅生长有关。

Effect of rare earth element Gd on microstructure and mechanical properties of Al-Si-Mg cast alloy

To systematically investigate the effect of rare earth element Gd on microstructure and mechanical properties of Al-Si-Mg(A357) cast alloy, A357 alloys with different Gd additions were investigated by OM, SEM, EPMA, XRD, DSC, TEM and tensile test. The results show that the addition of Gd can refine the grain and secondary dendrite arm spacing of the alloy. Besides, Gd refines eutectic Si, instead of transforming its morphology. The mechanical properties are improved by Gd addition due to obvious refinement of the grain and eutectic Si, the reduction of secondary dendrite arm spacing. For the A357-0.5Gd(mass fraction/%) alloy, the ultimate tensile strength(UTS) is 355MPa, which is 37MPa higher than that of the alloy without Gd. However, when increasing the Gd mass fraction to 1.0%, the formation of a high number of coarse Al2Si2 Gd results in the decrease of the mechanical properties of the alloy.The refinement mechanism of Gd was investigated.Combined with the TEM observation, it can be inferred that the Gd addition produces fewer twins but some nanosized particles within eutectic Si. The density of twins is not high enough to cause the morphology transition of eutectic Si. The refinement effect of Gd on eutectic Si may be attributed to the increase of constitutional overcooling and the formation of nanosized particles, inhibiting the growth of eutectic Si during solidification.